Studies were undertaken to determine the presence of retroviruses detectable by cell co-cultivation and virus induction methods in fibroblastic cultures of wild bats and kidney epithelial cultures of a woodchuck. Attempts were made to recombine (rescue) cellular 'sarc' genes of normal cat fibroblastic cells with feline leukemia virus by induction with a halogenated pyrimidine, 5-iododeoxyuridine. Studies were undertaken to molecularly clone the SM strain of feline sarcoma virus.